This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-136088, filed May 9, 2000; and No. 2000-233351, filed Aug. 1, 2000; and No. 2000-337966, filed Nov. 6, 2000, the entire contents of all of which are incorporated herein by reference.
The present invention relates to an optical encoder, particularly to an optical encoder as an optical displacement sensor which uses optical means for detecting a displacement amount of a precision mechanism.
Moreover, the present invention relates to an optical rotary encoder which uses optical means to detect a rotary angle.
As described in the beginning, as a prior art concerning a constitution of an optical encoder, first Jpn. Pat. Appln. KOKAI Publication No. 2000-205819 by the present inventor et al. will be described as a first prior example.
FIGS. 59A, 59B show constitution diagrams.
A scale 2 in which a first optical modulation region formed of a predetermined-period optical pattern generating a diffraction pattern (a transmission or reflective diffraction lattice scale in FIGS. 59A, 59B) is formed is irradiated with a laser mean emanating from a semiconductor laser 1 (or a surface emitting laser 10) as a coherence light source.
Moreover, the encoder is constituted such that a specific portion of the generated diffraction pattern is detected by either a photodetector 3 or a photodetector 3xe2x80x2.
Additionally, the coherence light source will also be referred to simply as a light source.
Additionally, when the coherence light source and photodetector 3 are disposed on the same side with respect to the scale 2, as shown in FIG. 59A, main axes 41, 42 of the light beam emitted from the semiconductor laser 1 (or the surface emitting laser 10) are inclined/arranged by angle xcfx86 with respect to a perpendicular line of a scale surface.
Operation of the sensor will next be described.
As shown in FIG. 59A, various constituting parameters will be defined as follows:
z1: length obtained by measuring a distance between the light source and the surface of the scale with a first light modulation region formed thereon on the light beam main axis;
z2: length obtained by measuring a distance between the surface of the scale with the first light modulation region formed thereon and a light receiving surface of the photodetector on the light beam main axis;
p1: pitch of the optical pattern in the first light modulation region on the scale;
p2: pitch of the diffraction pattern on the light receiving surface of the photodetector;
xcex8x: spread angle of the light beam of the light source with respect to a pitch direction of a diffraction lattice on the scale; and
xcex8y: spread angle of the light beam of the light source in a vertical direction with respect to the above xcex8x.
Additionally, the light beam spread angle means an angle formed by a pair of boundary lines 9 each having a direction in which a light beam intensity becomes xc2xd of a peak intensity.
Moreover, xe2x80x9cthe pitch of the optical pattern in the light modulation region on the scalexe2x80x9d means a space period of the pattern formed on the scale and having optical properties modulated.
Furthermore, xe2x80x9cthe pitch of the diffraction pattern on the light receiving surface of the photodetectorxe2x80x9d means a space period of an intensity distribution of the diffraction pattern generated on the light receiving surface.
According to a light diffraction theory, when z1 and z2 are in a specific relation satisfying the following equation (1), an intensity pattern substantially analogous to the scale diffraction lattice pattern is generated on the light receiving surface of the photodetector.
(1/z1)+(1/z2)=xcex/{k(p1)2}xe2x80x83xe2x80x83(1) 
In the equation, xcex denotes a light wavelength from the light source, and k denotes an integer.
In this case, the pitch p2 of the diffraction pattern on the light receiving surface can be represented using other constituting parameters as follows.
p2=p1(z1+z2)/z1 xe2x80x83xe2x80x83(2) 
When the scale 2 is displaced in the pitch direction of the diffraction lattice with respect to the light source, the diffraction pattern intensity distribution moves in a displacement direction of the scale 2 with the same space period being kept.
Therefore, a space period p21 of a light receiving area of the photodetector is set to the same value as that of p2. Every time the scale 2 moves by p1 in the pitch direction, a periodic strength signal is obtained from the photodetector. Therefore, the displacement amount of the scale 2 in the pitch direction can be detected.
Additionally, the above has been described on an assumption that the light beam extending to the scale from the light source has a constant spread angle (hereinafter referred to as xe2x80x9ca case of a spread beamxe2x80x9d).
Therefore, when the emanating beam from the light source is collimated to a parallel light by a lens, and the scale is irradiated with the light (hereinafter referred to as xe2x80x9ca case of a parallel beamxe2x80x9d), in the above equations (1) and (2), z1xe2x86x92∞ is assumed.
In this case, the equation (2) results in the following.
p2=p1 xe2x80x83xe2x80x83(2)xe2x80x2
Here, it is unnecessary to consider xe2x80x9cthe case of the parallel beamxe2x80x9d, but in xe2x80x9cthe case of the spread beamxe2x80x9d, as shown in FIGS. 59A, 59B, the light source and photodetector are disposed on the same side with respect to the scale 2 (hereinafter referred to as xe2x80x9ca reflective arrangementxe2x80x9d) such that z1=z2. In this case, even when a space gap between the scale 2 and the light source fluctuates, the pitch of the diffraction pattern on the light receiving surface does not change from the equation (2).
Moreover, in FIGS. 59A, 59B, the surface with the first light modulation region formed thereon is disposed substantially in parallel with the light receiving surface of the first photodetector. Additionally, the main axis of the first light beam is inclined/arranged with respect to the surface with the first light modulation region formed thereon only in a plane vertical to a predetermined period direction of the first light modulation region.
The diffraction pattern is regarded as a so-called shadow picture pattern. Therefore, even when the space gap between the scale 2 and the light source fluctuates because of the arrangement limiting a light beam inclined surface, a diffraction pattern intensity distribution curve is obtained as shown by curves 103, 104. Moreover, since the distribution does not move in a scale pitch direction, a displacement sensing is possible without being substantially influenced by the aforementioned gap fluctuation.
As described above, in both xe2x80x9cthe case of the parallel beamxe2x80x9d and xe2x80x9cthe case of spread beamxe2x80x9d it is an important point for the displacement sensing hardly influenced by the gap fluctuation to xe2x80x9cdispose the light beam main axis such that the axis is inclined with respect to the surface with the light modulation region formed thereon only in the plane vertical to the predetermined period direction of the light modulation regionxe2x80x9d.
Furthermore, in practical use, a light receiving group of the photodetector with a space period p20 is displaced at an interval of p21/4, four alternately arranged light receiving groups are formed, outputs Va, Vb, Vaxe2x80x2, Vbxe2x80x2 are obtained from light receiving elements of each group, and Va-Vaxe2x80x2, Vb-Vb are utilized as so-called A phase (sine wave) and B phase (cosine wave) outputs of the encoder, respectively.
Moreover, in the Jpn. Pat. Appln. KOKAI Publication No. 2000-205819, a laser beam intensity can be monitored by obtaining an operation sum of the respective outputs Va, Vb, Vaxe2x80x2, Vbxe2x80x2. Therefore, it is possible to correct an influence of a laser beam intensity change by an environment change and change with time to some degrees by feeding back a laser beam intensity change by the environment change and change with time so as to set the change to be constant, or by appropriately calculating A and B phase output signals and a signal of the operation sum of the respective outputs Va, Vb, Vaxe2x80x2, Vbxe2x80x2.
Therefore, in the first prior example, since there is hardly influence of the gap fluctuation between the scale and the head, the displacement of the scale in X direction can correctly be detected.
A second prior example similarly disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 2000-205819 is next shown in FIG. 60, FIG. 61.
In this example, a second track is formed in parallel with the pitch direction of a diffraction lattice track 21 on the scale.
In FIG. 60 the second track is a pattern 22 for detecting a reference position, and in FIG. 61 the second track is a diffraction lattice pattern 21xe2x80x2 having a period different from that of the other track.
These respective track patterns are irradiated with light beams from a surface emitting laser source, and a reflection or diffraction pattern is detected by optical detection means in the constitution.
In FIG. 60, the diffraction pattern formed by the diffraction lattice track 21 is detected by a light receiving element array group 32, and the A and B phase signals are outputted similarly as described above.
On the other hand, the light reflected by the pattern 22 for detecting the reference position is detected by a light receiving element 32 for detecting a light intensity.
For example, when a region shown by the pattern 22 has a reflectance larger than that of a peripheral portion, and only when the pattern 22 for detecting the reference position is irradiated with a second light beam, an output of the light receiving element 32 exceeds a predetermined value. Therefore, the reference position can be detected.
On the other hand, in FIG. 61, the diffraction patterns formed by the diffraction lattice tracks 21, 21xe2x80x2 are detected by light receiving element array groups 32, 32xe2x80x2, and two sets of A and B phase signals are outputted similarly as described above.
When the diffraction lattice pitches of the diffraction lattice tracks 21, 21xe2x80x2 are different from each other, an absolute position can be detected in a specific displacement range determined by a least common multiple of the respective pitches according to a vernier encoder principle in accordance with a difference.
Additionally, in the second prior example, as shown in FIG. 60, FIG. 61, the surface with the first light modulation region (diffraction lattice) formed thereon is disposed in parallel with the light receiving surface of the first optical detection means. However, the main axis of the first light beam is disposed to be inclined with respect to the surface with the first light modulation region formed thereon in a plane parallel to the predetermined period direction of the first light modulation region.
Therefore, in the aforementioned first prior example, since only the so-called A and B phase encoder outputs are obtained, it is possible to detect a relative displacement amount, but the absolute position cannot be detected.
On the other hand, in the aforementioned second prior example, a constitution in which a plurality of tracks different in the scale pattern are irradiated with the respective laser beams enables reference point detection and absolute position detection.
However, as shown in FIG. 60, FIG. 61, the main axis of the first light beam is disposed to be inclined with respect to the surface with the first light modulation region formed thereon in the plane parallel to the predetermined period direction of the first light modulation region.
Therefore, for the diffraction pattern, when the space gap between the scale 2 and the light source fluctuates, and the intensity distribution of the diffraction pattern moves in the scale pitch direction, outputs from the light receiving element arrays 32, 32xe2x80x2 and light receiving element 31 are outputted with the signal change by the displacement of the scale in an x direction and the influence by the gap fluctuation between the scale and the light source added thereto.
As a result, in the aforementioned second prior example, it is difficult to correctly detect only the displacement of the x direction.
Moreover, in the constitution of FIG. 60, when the light beam outputted from a second light beam emitting window 12 fluctuates by changes of ambient environments such as temperature and pressure, a signal level detected by the light receiving element 31 changes, and this hinders the correct reference position detection.
Furthermore, at present, utilized is a so-called encoder of an optical or magnetic type for detecting a linear displacement amount in a machine tool stage, three-dimensional measuring instrument, and the like, or for detecting a rotary angle in a servo motor, and the like.
The optical encoder is generally provided with a scale fixed to a member for detecting the displacement of the stage or the like, and a sensor head for detecting the displacement of the scale.
Here, a sensor head has the light source for irradiating the scale with the light beam, and the photodetector for detecting a diffraction light transmitted through the scale or reflected by the scale. A scale movement is detected in accordance with a change of a received light signal.
A typical optical encoder will be described as a third prior example.
FIG. 62 is a constitution diagram of an encoder using a surface emitting laser and reflective scale as one example of a small-sized low-cost laser encoder which requires no optical components such as a lens.
This laser encoder using the surface emitting laser and reflective scale is described, for example, in an article xe2x80x9cMicroencoder using Surface Emitting Semiconductor Laserxe2x80x9d (Eiji Yamamoto, Optics Vol. 27 No. 6 (1998)).
As shown in FIG. 62, an encoder 600 is constituted of a reflective scale 612 and a sensor head 614.
The sensor head 614 includes a surface emitting laser 616 and photodetector 618, both components are fixed to a base material 620, and a relative positional relation of the surface emitting laser 616 and photodetector 618 is maintained to be constant.
The scale 612 has a pattern in which a reflectance periodically changes in a direction vertical to a sheet surface.
This pattern is formed, for example, by patterning a high-reflectance material such as aluminum on the surface of a transparent substrate of glass or the like.
The scale 612 is interlocked with a stage (not shown) to reciprocate/move with respect to the sensor head 614 in the direction vertical to the sheet surface of FIG. 62, and the sensor head 614 detects this movement from a strength change of the light reflected from the scale 612.
The light beam emitted from the surface emitting laser 616 is reflected by the scale 612, and the reflected light is received by the photodetector 618.
For the pattern on the scale 612, since the reflectance of the pattern periodically changes in the direction vertical to the sheet surface, the displacement amount of the scale can be detected from the strength change of the received reflective light by the photodetector 618.
A laser encoder using a coherence light source and diffraction lattice scale will next be described as a fourth prior example.
FIG. 63 is a constitution diagram of the laser encoder using the coherence light source and diffraction lattice scale as one example of a small-sized low-cost encoder which requires no optical components such as the lens.
The laser encoder using the coherence light source and diffraction lattice scale is described, for example, in xe2x80x9cCopal: Rotary Encoder Catalogxe2x80x9d.
In a laser encoder 630, as shown in FIG. 63, a transmission type diffraction lattice scale 634 is irradiated with laser beams emitted from a semiconductor laser 632 as the coherence light source, and this generates a diffraction pattern 636 on a light receiving surface of a photodetector 640.
Definition of respective constituting parameters shown in FIG. 63 is the same as that of the aforementioned first prior example.
Moreover, as described above, according to a light diffraction theory, when z1, z2 defined as described above are in a specific relation satisfying the relation shown in the above equation (1), an intensity pattern analogous to the diffraction lattice pattern of the scale 634 is generated on the light receiving surface of the photodetector 640.
In this case, the pitch p2 of the diffraction pattern on the light receiving surface can be represented using the other constituting parameters as in the above equation (2).
Moreover, as described above, when the scale is displaced in the pitch direction of the diffraction lattice with respect to the light source, the diffraction pattern intensity distribution moves in the scale displacement direction with the same space period being kept.
Here, the photodetector 640 has a plurality of light receiving areas 642, the light receiving areas 642 are arranged in parallel with a moving direction of the scale 634 at the space period p20, and the space period p20 is equal to the diffraction lattice scale pitch p2.
Therefore, every time the scale 634 moves by p1 in the pitch direction, a strength signal periodically changing at the period p2 is obtained from the photodetector 640, and the displacement amount of the scale 634 in the pitch direction is detected.
Additionally, this type of optical encoder is of a high-precision, high-resolution, non-contact type, is superior in resistance to electromagnetic wave trouble, and has other characteristics.
Therefore, this type of optical encoder is utilized in various fields, and especially the encoder requiring high precision and high resolution is mainly of an optical type.
However, the aforementioned optical encoders according to the third and fourth prior examples have the following problems.
First, in the third prior example, an exclusive fixing base needs to be used to incline/dispose the light source with respect to the scale, assembling therefore becomes difficult, and this causes a cost rise.
Moreover, in the third prior example, a distance, that is, a so-called gap between the light source and the scale needs to be strictly adjusted. Otherwise, the reflected light from the scale is not incident upon a specific portion of the photodetector, and the signal strength and precision are adversely affected.
Furthermore, in the third prior example, when the light source is disposed in the vicinity of the light receiving area of the photodetector to miniaturize the sensor head, the reflected light beam from the scale is obstructed by a chip end of the surface emitting laser as the light source, and does not reach the light receiving area.
Moreover, in the third prior example, it is further necessary to further enlarge an attachment angle of the light source in order to avoid this situation. Therefore, miniaturization and cost reduction are difficult.
Furthermore, in the constitution as shown in the fourth prior example, expensive optical components such as a slit are used. Additionally, since these optical components need to be assembled with a high precision, the miniaturization and cost reduction are remarkably difficult.
Additionally, in both the third and fourth prior examples, when information is read from the scale with a plurality of optical patterns formed thereon, it is necessary to use the same number of light sources as that of optical patterns, and to split the beam using a beam splitter, and the like. The miniaturization and cost reduction are further difficult.
An object of the present invention is to provide a reference point detection function and absolute position detection function, and to provide an optical encoder which can correctly detect a displacement of a scale in an x direction with little influence of a fluctuation of a gap between the scale and a head, and an optical encoder which enables a stable reference point detection and absolute position detection even with a change of ambient environment.
Another object of the present invention is to provide an optical encoder in which a light source does not have to be inclined with respect to the scale, no strict precision is required for adjustment of the gap between the scale and the head, and miniaturization and cost reduction are therefore realized.
Further object of the present invention is to provide an optical rotary encoder which does not cause an output drop.
To achieve the aforementioned objects, according to a first aspect of the present invention, there is provided xe2x80x9can optical encoder comprising:
a first coherence light source for emitting a first light beam;
a scale which can be displaced to cross the first light beam emitted from the first light source, and which has a first light modulation region formed of a predetermined-period optical pattern irradiated with the first light beam to generate a diffraction pattern;
first photodetection means comprising a light receiving surface for receiving the first light beam transmitted via the first light modulation region of the scale, and a single light receiving element group or a plurality of light receiving element groups formed at a predetermined interval so as to detect a predetermined portion of the diffraction pattern generated by irradiating the first light modulation region of the scale with the first light beam in the light receiving surface;
a second light source for emitting a second light beam;
a second light modulation region for modulating optical properties of the second light beam emitted from the second light source; and
second photodetection means for receiving the second light beam with the optical properties modulated by the second light modulation region,
wherein the first light modulation region and the second light modulation region are positioned in series in a moving direction of the scale, and the second light modulation region moves integrally with the first light modulation region with movement of the scalexe2x80x9d.
Moreover, to achieve the aforementioned objects, according to a second aspect of the present invention, there is provided xe2x80x9can optical encoder comprising:
a movable scale with a predetermined-period optical pattern formed therein;
a coherence light source for substantially vertically irradiating an optical pattern surface of the movable scale with a light beam having a predetermined shape; and
photodetection means for receiving the light beam emitted from the light source and reflected by the optical pattern, and detecting a diffraction pattern generated by the optical pattern on a light receiving surfacexe2x80x9d.
Furthermore, to achieve the aforementioned object, according to a third aspect of the present invention, there is provided xe2x80x9can optical rotary encoder comprising:
a coherence light source;
a disc-shaped scale which rotates to cross a light beam emitted from the coherence light source, and in which a radial optical pattern with a predetermined angle period to be irradiated with the light beam is formed; and
a photodetector having a plurality of light receiving areas, arranged in a radial form from a circle center, for receiving the light beam transmitted via the optical pattern and detecting a bright/dark pattern generated by the optical pattern,
wherein for the bright/dark pattern, a plurality of patterns are formed such that a bright portion is formed at the same angle on a circumference having a different distance from the circle center, and the bright/dark pattern comprises a first bright/dark pattern group and a second bright/dark pattern group in which bright portions are formed at different angles in a circumferential direction, and
for an effective detection sensitivity of each light receiving area of the photodetector, an effective light receiving sensitivity of a portion with the bright portion of either the first bright/dark pattern group or the second bright/dark pattern group formed therein is higher than the light receiving sensitivity of other portions of the light receiving areaxe2x80x9d.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.